Previous studies indicate that HCMV infection is a risk factor for TA. Viral-specific CD4 and CD8 T cell responses appear to be critical for maintenance of HCMV latency. HCMV genomes or gene products have been detected in the cells of the coronary vasculature in patients with TA. We hypothesize that such local infection could enhance alloreactivity via direct effects of the virus on cell function as well as by recruiting recipient inflammatory cells. The studies of Specific Aim 1, longitudinal assays will follow viral load and HCMV-specific effector cells (circulating CD4 and CD8 T lymphocytes) during primary infection (donor +/ recipient -), latency with reactivation or reinfection (recipient +/donor - or recipient +/donor +), and latency without reactivation (non-transplant HCMV + healthy individuals). An inverse correlation is expected between the risk of progression to TA and the frequency of HCMV-specific T cells capable of producing cytokines (IL-2 and IFN-gamma), and TNF family ligands (TNF-cc, CD40-ligand, fas-ligand). The risk of progression to TA may directly correlate with an increased frequency of T cells producing Th2-type cytokines, IL-4, IL-10, and IL-13, which are expected to be ineffective in limiting viral replication. The studies of specific aim 2 will follow the frequency and function of CD4 and CD8 T cells recognizing pp65 epitopes, a widely recognized indicator viral antigen, and determine whether the HCMV-specific memory/effector response is reduced in heart transplant patients who progress to TA compared to these cells in healthy adults. Patients who are infected with HCMV and are HLA-A.2 positive will be evaluated for the presence of CD8 T cells that bind the HCMV pp65 495.503- class I tetramer and cell activation function, based on CD69 expression and IFN-gamma production and basal perforin expression. Similar studies will be performed for CD4 T cell responses with HLA-DR/pp65 peptide tetramers, if these become available. We anticipate that transplant patients will have a decreased frequency of memory/effector CD4 and CD8 T cells with specificity for pp65-derived peptides, and that a greater proportion of cells with this antigenic specificity will have reduced function, based on cytokine secretion, compared to healthy control HCMV seropositive individuals. We expect that the risk of TA would be lower in patients with more rapid control of systemic or local HCMV infection due to robust HCMV-specific adaptive immune responses. It is also plausible that in cases of direct HCMV infection of the graft vasculature, these delayed adaptive immune responses might ultimately contribute to the later development of TA. Together, these studies should provide insight into human CD4 and CD8 memory/effector T cell responses following primary and recurrent HCMV infection in cardiac transplant recipients and their relationship with the evolution of TA. They should also provide insight into the relationship between HCMV infection and allograft rejection in other transplant contexts.